View source for Williams's p + 1 algorithm

{{DISPLAYTITLE:Williams's ''p''&nbsp;+&nbsp;1 algorithm}}
In [[computational number theory]], '''Williams's ''p''&nbsp;+&nbsp;1 algorithm''' is an [[integer factorization]] algorithm, one of the family of [[algebraic-group factorisation algorithms]].  It was invented by [[Hugh C. Williams]] in 1982. 

It works well if the number ''N'' to be factored contains one or more prime factors ''p'' such that ''p'' + 1 is [[smooth number|smooth]], i.e. ''p'' + 1 contains only small factors. It uses [[Lucas sequence]]s to perform exponentiation in a [[quadratic field]].

It is analogous to [[Pollard's p - 1 algorithm|Pollard's ''p''&nbsp;&minus;&nbsp;1 algorithm]].

==Algorithm==
Choose some integer ''A'' greater than 2 which characterizes the [[Lucas sequence]]:

:<math>V_0=2, V_1=A, V_j=AV_{j-1}-V_{j-2}</math>

where all operations are performed modulo ''N''.

Then any odd prime ''p'' divides <math>\gcd(N,V_M-2)</math> whenever ''M'' is a multiple of <math>p-(D/p)</math>, where 
<math>D=A^2-4</math> and 
<math>(D/p)</math> is the [[Jacobi symbol]].

We require that <math>(D/p)=-1</math>, that is, ''D'' should be a [[quadratic non-residue]] modulo ''p''.  But as we don't know ''p'' beforehand, more than one value of ''A'' may be required before finding a solution.  If <math>(D/p)=+1</math>, this algorithm degenerates into a slow version of [[Pollard's p - 1 algorithm|Pollard's p&nbsp;&minus;&nbsp;1 algorithm]].

So, for different values of ''M'' we calculate <math>\gcd(N,V_M-2)</math>, and when the result is not equal to 1 or to ''N'', we have found a non-trivial factor of ''N''.

The values of ''M'' used are successive factorials, and <math>V_M</math> is the ''M''-th value of the sequence characterized by <math>V_{M-1}</math>.

To find the ''M''-th element ''V'' of the sequence characterized by ''B'', we proceed in a manner similar to left-to-right exponentiation:
  x=B           
  y=(B^2-2) mod N     
  for each bit of M to the right of the most significant bit
    if the bit is 1
      x=(x*y-B) mod N 
      y=(y^2-2) mod N 
    else
      y=(x*y-B) mod N 
      x=(x^2-2) mod N 
  V=x

==Example==
With ''N''=112729 and ''A''=5, successive values of <math>V_M</math> are:

: V<sub>1</sub> of seq(5) = V<sub>1!</sub> of seq(5) = 5

: V<sub>2</sub> of seq(5) = V<sub>2!</sub> of seq(5) = 23

: V<sub>3</sub> of seq(23) = V<sub>3!</sub> of seq(5) = 12098

: V<sub>4</sub> of seq(12098) = V<sub>4!</sub> of seq(5) = 87680

: V<sub>5</sub> of seq(87680) = V<sub>5!</sub> of seq(5) = 53242

: V<sub>6</sub> of seq(53242) = V<sub>6!</sub> of seq(5) = 27666

: V<sub>7</sub> of seq(27666) = V<sub>7!</sub> of seq(5) = 110229.

At this point, gcd(110229-2,112729) = 139, so 139 is a non-trivial factor of 112729. Notice that p+1 = 140 = 2<sup>2</sup> &times; 5 &times; 7. The number 7! is the lowest factorial which is multiple of 140, so the proper factor 139 is found in this step.

Using another initial value, say ''A'' = 9, we get:

: V<sub>1</sub> of seq(9) = V<sub>1!</sub> of seq(9) = 9

: V<sub>2</sub> of seq(9) = V<sub>2!</sub> of seq(9) = 79

: V<sub>3</sub> of seq(79) = V<sub>3!</sub> of seq(9) = 41886

: V<sub>4</sub> of seq(41886) = V<sub>4!</sub> of seq(9) = 79378

: V<sub>5</sub> of seq(79378) = V<sub>5!</sub> of seq(9) = 1934

: V<sub>6</sub> of seq(1934) = V<sub>6!</sub> of seq(9) = 10582

: V<sub>7</sub> of seq(10582) = V<sub>7!</sub> of seq(9) = 84241

: V<sub>8</sub> of seq(84241) = V<sub>8!</sub> of seq(9) = 93973

: V<sub>9</sub> of seq(93973) = V<sub>9!</sub> of seq(9) = 91645.

At this point gcd(91645-2,112729) = 811, so 811 is a non-trivial factor of 112729. Notice that p-1 = 810 = 2 &times; 5 &times; 3<sup>4</sup>. The number 9! is the lowest factorial which is multiple of 810, so the proper factor 811 is found in this step. The factor 139 is not found this time because p-1 = 138 = 2 &times; 3 &times; 23 which is not a divisor of 9!

As can be seen in these examples we do not know in advance whether the prime that will be found has a smooth p+1 or p-1.

==Generalization==
Based on [[Pollard's p - 1 algorithm|Pollard's ''p''-1]] and Williams's ''p''+1 factoring algorithms, Eric Bach and Jeffrey Shallit developed techniques to factor ''n'' efficiently provided that it has a prime factor ''p'' such that any ''k''<sup>th</sup> [[cyclotomic polynomial]] Φ<sub>''k''</sub>(''p'') is [[Smooth number|smooth]].<ref>{{cite journal | first1=Eric | last1=Bach | first2=Jeffrey | last2=Shallit | title=Factoring with Cyclotomic Polynomials | publisher=[[American Mathematical Society]] | journal=Mathematics of Computation | volume=52 | issue=185 | year=1989 | pages=201–219 | jstor=2008664 |url=http://www.ams.org/journals/mcom/1989-52-185/S0025-5718-1989-0947467-1/S0025-5718-1989-0947467-1.pdf |doi=10.1090/S0025-5718-1989-0947467-1}}</ref>
The first few cyclotomic polynomials are given by the sequence Φ<sub>1</sub>(''p'') = ''p''-1, Φ<sub>2</sub>(''p'') = ''p''+1, Φ<sub>3</sub>(''p'') = ''p''<sup>2</sup>+''p''+1, and Φ<sub>4</sub>(''p'') = ''p''<sup>2</sup>+1.

==References==
{{reflist}}

*{{Citation |last=Williams |first=H. C. |year=1982 |title=A p+1 method of factoring |journal=Mathematics of Computation |volume=39 |issue=159 |pages=225–234 |doi=10.2307/2007633 |mr=0658227 }}

==External links==
* [http://www.mersennewiki.org/index.php/P_Plus_1_Factorization_Method P Plus 1 Factorization Method], MersenneWiki.

{{number theoretic algorithms}}

[[Category:Integer factorization algorithms]]